Declarative extension plugin for window decorations

The generic QML components from Aurorae are split out into an
own declarative plugin. In addition two new helper classes are
added to this plugin:
* A ColorHelper to map a few function of KColorSheme and making
it possible to actually work with colors in QML. The need
emerged from trying to port Plastik to QML which makes strong
use of color shading.
* A DecorationOptions class which is a wrapper around KWin's
KDecorationOptions but in a more useable way for QML. The
various options are provided as properties and the value of
the properties changes automatically depending on whether the
decoration is active or inactive.

Aurorae itself is not yet adjusted to these changes, but it
should also be adjusted as some of the options are currently
exported in the factory and the factory is injected into the
Aurorae QML decoration.